If the router is doing nothing more than connecting to a T1, I see no value in using anything with more horsepower than a 1760, but at the same time, you don't need the voice codecs in the 1760. Keeping them both the same also has advantages.
If you're connecting to a second ISP, how are you handling the redundancy without BGP? Using BGP will require more memory and horsepower than either of these two routers...